Flavonoid–drug interactions: Effects of flavonoids on ABC transporters

Abstract

Flavonoids are present in fruits, vegetables and beverages derived from plants (tea, red wine), and in many dietary supplements or herbal remedies including Ginkgo Biloba, Soy Isoflavones, and Milk Thistle. Flavonoids have been described as health-promoting, disease-preventing dietary supplements, and a high intake of flavonoids has been associated with a reduced risk of cancer, cardiovascular diseases, osteoporosis and other age-related degenerative diseases. Due to an increased public interest in alternative medicine and disease prevention, the use of herbal preparations containing high doses of flavonoids for health maintenance has become very popular, raising the potential for interactions with conventional drug therapies. This review will summarize the current literature regarding the interactions of flavonoids with ATP-binding cassette (ABC) efflux transporters, mainly P-glycoprotein, MRP1, MRP2 and BCRP and discuss the potential consequences for flavonoid–drug transport interactions.

Introduction

Flavonoids are a large class of naturally occurring compounds widely present in the green plant world with more than 6500 different compounds described (Harborne and Williams, 2000). In general, these compounds possess a skeleton of a chromane ring with an additional aromatic ring attached at position 2, 3 or 4 (Fig. 1). Based on different substitution and the oxidation status of ring C, flavonoids can be classified into several subclasses including flavones, flavonols, flavonones, flavanols, isoflavones and chalcones (Fig. 1). Flavonoids are also an integral component of our common diet and they are particularly abundant in vegetables, fruits and plant-derived beverages such as wine and tea. For example, grapefruit juice has been shown to contain nearly 200–850 mg/L of total flavonoids, among which naringin is the most abundant (145–638 mg/L) (Ross et al., 2000b) while orange juice mainly contains hesperidin and its content has been shown to be 200–450 mg/L (Erlund et al., 2001, Manach et al., 2003). Soy foods are rich sources for the isoflavones genistein and daidzein and every gram of dry bean contains about 1 mg of these compounds (Reinli and Block, 1996). Green tea and red wine are rich in catechins including epicatechin, epigallocatechin, epicatechin-3-gallate and epigallocatechin-3-gallate, and the content of these compounds in green tea can be as high as 1000 mg/L (Lee et al., 1995). The daily intake of total flavonoids in the U.S. diet has been estimated to be 1 gram (Kuhnau, 1976), but it is likely that this figure may be an overestimation and the actual consumption could be substantially lower.

In nature, most flavonoids exist as glycosides, but the aglycones are generally believed to be absorbed from the gastrointestinal tract after ingestion due to their higher hydrophobicity. The glycosides undergo hydrolysis by glycosidases present in the cells of the gastrointestinal mucosa or secreted by the colonic microflora. This is consistent with the observation that the time to achieve maximum plasma concentrations after oral dosing of genistin or daidzin (glycosides of genistein and daidzein, respectively) is much longer compared to that after oral dosing of the corresponding aglycones (9 h versus 2 h), and that no detectable glycosides were found in plasma (Setchell et al., 2001). Additionally, quercetin but not quercetin glucosides could be detected in ileostomy fluids of patients after ingesting an onion meal containing 10.9 to 51.6 mg of quercetin glucosides with only a trace amount of quercetin, indicating that quercetin glucosides were efficiently hydrolyzed to the aglycone quercetin in the intestine (Walle et al., 2000). However, some controversy exists; the bioavailability of quercetin glucosides from onions was found to be significantly higher than that of the aglycone (Hollman et al., 1995, Hollman et al., 1996), suggesting that glycosides may be also absorbed to some extent. Indeed, quercetin glucosides have been shown to be substrates of the intestinal sodium-dependent glucose transporter (SGLT-1) (Walgren et al., 2000b, Wolffram et al., 2002), implying that these glycosides could be absorbed with the help of SGLT-1. Overall, definitive information regarding flavonoid absorption and bioavailability requires further investigation. Flavonoids undergo extensive intestinal and hepatic glucuronidation and sulfation (Chen et al., 2003, Liu and Hu, 2002) and the predominant circulating chemical species in vivo after an oral dose are these conjugative metabolites. The parent compound concentrations in the systemic circulation are usually below the micromolar range, but their presence is persistent, probably due to the enterohepatic recycling of the conjugated metabolites (Setchell et al., 2001, Walle et al., 2001). However, when a large dose is given intravenously, bypassing first pass metabolism, the plasma concentrations of the parent compounds could reach more than 100 μM (Ferry et al., 1996). Flavonoid metabolism by cytochrome P450 has been observed in in vitro biotransformation studies using rat and human liver microsomes (Hu et al., 2003, Nielsen et al., 1998), but its in vivo significance is, in general, unknown.

In recent years, there has been a resurgence of scientific interest in flavonoids with more than 2000 publications per year containing “flavonoids” as a key word. This is due to the association of these compounds with a wide range of health promoting effects. Numerous studies have indicated that flavonoids have anti-oxidant, anti-carcinogenic, anti-viral, anti-inflammatory and anti-estrogenic or estrogenic activities (Havsteen, 2002, Middleton et al., 2000). High intake of flavonoids has been linked with reduced risk of cancer, cardiovascular diseases, osteoporosis and other age-related degenerative diseases (Havsteen, 2002, Hertog et al., 1993, Huxley and Neil, 2003, Keli et al., 1996, Lee et al., 1991, Middleton et al., 2000, Potter et al., 1998). For example, much of the interest in flavonoids has recently been focused on their anti-cancer properties. Epidemiological studies have suggested an association between flavonoid intake and a reduced risk of certain cancers. The lower rate of breast cancer incidence and mortality observed in Japanese women and women of Japanese origin living in Hawaii has been attributed to their high consumption of the traditional soy product-rich Japanese diet; soy products contain isoflavonoids (Messina et al., 1994). Additionally, a reduced risk of breast cancer incidence has also been associated with a high intake of daidzein and genistein in a German case-control study (Linseisen et al., 2004). In animal studies, administration of flavonoids has been shown to prevent the development and growth of various types of chemical carcinogen-induced or transplanted tumors (Buchler et al., 2003, Kohno et al., 2002, Rice et al., 2002). The proposed mechanisms for these cancer prevention effects are multifaceted, including their anti-oxidant activities, their effects on signal transduction pathways involved in cell proliferation and angiogenesis, as well as their modulation of aromatase activity, a key enzyme involved in estrogen biosynthesis, and the enzymes required for metabolic activation of procarcinogens and the detoxification of carcinogens (Kellis and Vickery, 1984, Middleton et al., 2000)}. In fact, synthetic flavonoid derivatives, flavone acetic acid and flavopiridol, have been evaluated in Phase II clinical trials for their anti-cancer activities (Aklilu et al., 2003, Siegenthaler et al., 1992). In addition to their variety of health-promoting activities, flavonoids themselves are believed to have no or little toxicity and have a long history of human consumption (Middleton et al., 2000). Very large doses of these compounds (up to 500 mg/kg) have been administered to animals, with little or no toxicity reported.

Due to their wide range of health-beneficial activities and their remarkable safety records, numerous herbal preparations containing either flavonoid glycosides or aglycones, including the widely used milk thistle (Silybum marianus) and red clover (trifolium pratense) extracts, are now marketed in various formulations as dietary supplements. The intake of flavonoids after taking these herbal products is likely very high. For example, the recommended dose of chrysin supplements is 1–4 capsules daily and each capsule contains 500 mg chrysin (http://www.herbsmd.com/shop/xq/asp/pid.1646/qx/productdetail.asp); the recommended dose of quercetin supplements is one capsule (620 mg quercetin) daily (http://www.viable-herbal.com/singles/herbs/s914.htm). Daflon 500 is a phlebotonic drug and a vascular protecting agent containing 450 mg diosmin and 50 mg hesperidin per tablet; the suggested dose is up to 6 tablets a day (http://www.medmart.worldmedic.com/Domestic/Memberphar/servier/daflon500.htm) Therefore, the flavonoid concentration, at least in the intestine, could be very high after ingestion of some food and especially, flavonoid-containing supplements, suggesting a potential for drug interactions. Moreover, due to an increased public interest in alternative medicine and disease prevention, the use of herbal preparations for health maintenance has become more popular, and it has been estimated that herbal products are ingested by about 10% or more of the general population and 30–70% of individuals with specific disease states (Duggan et al., 2001, Ni et al., 2002). Thus, the consumption of large doses of flavonoids is frequent, increasing the risk of flavonoid–mediated pharmacokinetic interactions with conventional medication. This concern is relevant, because increasing evidence has indicated that significant or even life-threatening interactions between flavonoid-containing products and conventional drugs can occur. For example, coadministration of grapefruit juice, which contains a large amount of the flavonoid naringin, significantly increased the oral bioavailability of felodipine (Bailey et al., 1993), nimodipine (Fuhr et al., 1998), cyclosporine (Ducharme et al., 1995) and saquinavir (Kupferschmidt et al., 1998), and decreased the oral bioavailability of fexofenadine (Dresser et al., 2002) in human subjects. Silymarin has been reported to increase the clearance of metronidazole and its major metabolite, hydroxy-metronidazole (Rajnarayana et al., 2004). In animals, naringin increases the oral bioavailability of quinine in rats (Zhang et al., 2000) while baicalin and its aglycone baicalein both increase the oral bioavailability of cyclosporine in rats (Lai et al., 2004) and flavone and quercetin can increase the oral bioavailability of paclitaxel in rats (Choi et al., 2004a, Choi et al., 2004b). On the contrary, quercetin and phellamurin decrease the oral bioavailability of cyclosporine in pigs and/or rats (Chen et al., 2002, Hsiu et al., 2002). Considering this accumulating evidence for flavonoid–drug interactions from clinical and animal studies, the frequent presence of high flavonoid content in dietary supplements, and the increasing popularity of many flavonoid-containing herbal products, which do not require FDA approval in the U.S. for marketing, a careful evaluation of the interaction of commonly ingested flavonoids with molecular mechanisms determining drug disposition including absorption, distribution, metabolism and elimination becomes important.

Xenobiotic and endobiotic agents must pass through various biological membrane systems for their absorption, distribution, metabolism and elimination, as well as for binding to their intracelullar targets such as enzymes or receptors to exert their biological functions. The biological membrane system is a lipid bilayer system embedded with numerous proteins including many transporters. Thus, the activities of these transporters are expected to be important determinants for the pharmacokinetics and pharmacodynamics of many important drugs especially those hydrophilic compounds. Knowledge about these transporters has been gained over the past decade, including their functional characteristics, substrate specificities, and their specialized tissue distribution and subcellular localization. Transport proteins play a key role in determining drug absorption, elimination as well as drug entry into some pharmacologically important compartments, such as brain. Therefore, understanding the interaction of flavonoids with these drug transporters will help us understand and predict potential flavonoid–drug interactions. Among these transporters, a group of so called ABC (ATP-binding cassette) transporters including P-glycoprotein, multidrug resistance associated proteins (MRPs) and breast cancer resistance protein (BCRP), have attracted special attention because of their involvement in developing multidrug resistance (MDR) (Litman et al., 2001) and their demonstrated significance in pharmacokinetics and pharmacodynamics. P-glycoprotein, MRPs and BCRP are all plasma membrane efflux transporters, pumping their substrates out of the cells using the energy derived from ATP hydrolysis (Litman et al., 2001). The focus of this overview is to summarize the current findings about the interactions of flavonoids with these efflux transporters, mainly P-glycoprotein, MRP1, MRP2 and BCRP and discuss the potential consequences for flavonoid–drug interactions.